The present disclosure generally relates to systems and methods for generating and distributing electrical power, and more particularly to such systems and methods in which voltage at the remote electrical load is communicated to the generator via a serial communication network.
Typically, a generator is a rotary electric machine of well-known type having a stator surrounding a rotor driven through a belt or shaft by a prime mover (e.g., an engine) to electromagnetically induce electrical current in conductive windings of the stator, whereby mechanical power is converted into electrical power. The stator includes phase coils coupled in a Delta or Wye configuration. A generator may be a DC type that produces direct current or an AC type that produces an alternating current, the latter type also referred to as an alternator.
Where used to charge a battery (or multiple batteries) that powers an electrical system, the alternator output is a rectified DC voltage. The stator is electrically coupled to the rectifier, which delivers the alternator output to the alternator's B+ terminal. The voltage at the alternator B+ terminal is the alternator's controlled voltage setpoint or internal control point voltage, and also referred to herein as VCONTROL. The B+ terminal is electrically connected via a cable to the battery, which is the primary component of the alternator load. The voltage drop between the alternator internal control point voltage at the B+ terminal (i.e., VCONTROL) and the actual battery voltage, VBAT, can change with battery cable resistance, and thus with alternator output current.
Parallel alternator systems, wherein multiple alternators are electrically connected to each other in parallel, may be adapted for use in mobile installations, and may be the primary power source for charging batteries that provide electrical power for various types of vehicles, such as over-the-road tractors or large buses for example. Parallel alternator systems, particularly those used in vehicles, typically employ a single engine to drive the multiplicity of alternators. For example, the single engine of an over-the-road tractor or large bus drives each of a multiplicity of parallel-connected alternators mounted to the engine and driven by the engine crankshaft through one or more belts. The multiple alternators may be identical to each other, and may be driven at a common speed that is a ratio of the engine crankshaft speed. In a parallel alternator system, the entire load is shared by all of the parallel-connected alternator units operating in the system and load sharing between these alternator units is typically done to ensure they all contribute the same power toward the system load, or so that they all share the same voltage setpoint. The individual alternator units operating in parallel systems are typically of smaller capacities, and may be of identical or variable output relative to each other.
Regardless of whether one alternator unit or a plurality of parallel-connected alternator units is utilized by the battery charging system, the output of the stator windings of each alternator unit providing power to the system is normally controlled by either a single voltage regulator common to all alternator units in the system, or a dedicated voltage regulator for that respective alternator unit. Relative to each alternator unit, the alternator regulator (whether common to all alternator units in the system, or dedicated to that alternator unit) is configured to control the excitation current to a field coil carried by the rotor of the alternator unit. The regulator includes a field driver circuit configured to deliver the electric excitation current signal to the rotor field coil at a switching frequency. Although some alternators utilize variable frequency field drivers that provide current pulses to their rotor field coils at varying frequencies, vehicle alternators have traditionally utilized fixed frequency field drivers. In these alternators, a field driver circuit provides pulses of current to the rotor field coil at a fixed frequency. The rotor field coil receives a signal indicative of a predetermined duty cycle from the regulator. The strength of the alternator rotor's moving magnetic field, which induces alternating current flow in the stator windings of the surrounding stator is controlled by the voltage regulator(s). The alternating stator output current is rectified to generate a DC alternator output voltage, which is the alternator voltage setpoint, VCONTROL.
In general, when more excitation current is provided by the regulator to the rotor field coil, the output voltage setpoint of the alternator at its B+ terminal increases, and when less excitation current is provided to the rotor field coil, the output voltage setpoint of the alternator decreases. The alternator's DC voltage setpoint at the B+ terminal directly correlates to the rotor field coil excitation current or rotor field duty cycle, which is regulated in response to the electrical load on the alternator. The correlated rotor field excitation current or duty cycle, and alternator load, are commonly expressed as a percentage between 0% and 100%. Thus, the alternator generates an output voltage, VCONTROL, having a magnitude based on the duty cycle of the regulated excitation current signal applied to the rotor field coil.
Typically, an alternator has its own dedicated digital microcontroller (the “alternator controller”) that controls the operation of the alternator unit and may include both the regulator and the rectifier. The alternator controller may be a plug and play device. In a system of parallel-connected alternators, the alternator controllers of the system's alternators cooperate in the operation of the overall battery charging system, which is controlled by a system controller. Moreover, the plurality of alternator controllers may coordinate among themselves to control the system or designate a system controller that is internal to one alternator. Alternatively, the system controller may be an external electronic control unit (“ECU”) in communication with the alternator controllers but located remotely from the alternators.
The regulator's field driver circuit is controlled by the alternator controller and is configured to control the excitation current provided to the rotor field coil. The field driver circuit may include a MOSFET transistor configured to control the electric current delivered to the rotor field coil. The transistor is switchable between an on-state and an off-state at the switching frequency. Transistor switching of each of the parallel-connected alternators or the sole alternator of the charging system, as the case may be, is controlled by a respective alternator controller.
The alternator controller controls the current output of the regulator to the rotor field coil by delivering control signals to the gate of the field driver circuit transistor. These control signals switch the transistor on and off such that the regulator output field voltage is provided to the rotor field coil as a pulse signal to regulate excitation current. The field voltage signal has a pulse duration τ, and a pulse period T at which the pulses repeat. The rotor field duty cycle, FROTOR, is calculated as τ/T. Depending on the inputs received, the alternator controller may adjust its rotor field duty cycle FROTOR in an attempt to control the alternator output voltage setpoint by increasing or decreasing the pulse duration T of the field voltage signal. The commanded duty cycle of the alternator (i.e., its rotor field duty cycle), which can range between 0% and 100%, thus directly corresponds to the stator output voltage, which is rectified to provide the alternator voltage setpoint (VCONTROL) at the alternator B+ terminal as a DC voltage. Thus, the rotor field duty cycle, FROTOR, the rotor excitation current, IROTOR, and the alternator voltage setpoint, VCONTROL, directly correspond to each other.
The alternator B+ terminal is connected through a battery cable to the positive terminal of a battery for charging the battery, which is the primary load on the charging system. In the case of a parallel-connected alternator system, the connection between the B+ terminals of the parallel-connected alternators and the battery is through an intermediate voltage bus which is essentially part of the battery cable between each alternator B+ terminal and the battery. When the battery charging system is under load, i.e., when electrical power from the alternator is delivered to the battery, current flows from the alternator B+ terminal through the battery cable. Consequently, voltage losses occur within the battery cable, which can result in voltage drop from the voltage level at the alternator B+ terminal being realized at the battery. The battery cable voltage losses generally increase linearly with alternator load above the alternator's substantially unloaded state (i.e., at the 0% alternator load level), in which no battery cable losses occur.
Some prior battery charging systems and alternators thereof have external remote sensing capabilities, which utilize a small gauge sensing wire external to the alternator and connected to the battery, through which the system reads the actual voltage at the battery, VBAT,COMP. The regulator of the remote sensing alternator of this system has an input pin to which the system's external sensing wire is also connected. The alternator regulator receives as an analog signal, and thus senses, the battery voltage VBAT,COMP for use in adjusting the alternator's output voltage setpoint. The adjustments to the alternator's voltage setpoint offset or compensate for voltage losses occurring in the battery cable, and ensures that the battery voltage VBAT,COMP is maintained at a desired level over the range of alternator loads.
In battery charging systems having external remote sensing capabilities, the additional, compensating portion of the output voltage setpoint provided at the alternator's B+ terminal while the alternator is under load forces current into the battery at a relatively faster rate, thereby decreasing the charging time to reach full battery charge. Typically, the regulated adjustments to the alternator voltage setpoint are such as to maintain VBAT,COMP at a constant level equivalent to the alternator's output voltage setpoint at 0% alternator load, VSet0.
In FIG. 1, which relates to comparable example 24-volt charging systems, line 20 shows that between 0% and 100% of alternator load, a prior system having external remote sensing capabilities maintains the voltage at the battery, VBAT,COMP, at a substantially constant level equal to VSet0, which in the examples discussed herein, is 28.3 volts. In the illustrated example, the charging system's external remote sensing capabilities adjust VCONTROL,COMP to offset or compensate the voltage drop that occurs in the battery cable between the alternator B+ terminal and the battery, to maintain the battery voltage, VBAT,COMP, at a constant level of 28.3 volts, a voltage equal to the alternator's prescribed VSet0 level.
Internally of an alternator used in a charging system having external remote sensing capabilities, the alternator output voltage setpoint at the alternator B+ terminal, VCONTROL,COMP, linearly increases with alternator load to compensate for the linearly increasing voltage losses in the battery cable connecting the alternator B+ terminal and the battery, as indicated by line 22 of FIG. 2. VCONTROL,COMP ranges linearly along line 22 from VSet0, which is 28.3 volts at 0% alternator load, to 28.5 volts at 100% alternator load. The linearly increasing alternator voltage output VCONTROL,COMP offsets or compensates for the linearly increasing battery cable voltage losses, which range from 0 volts at 0% load to 0.2 volts at 100% load, and maintains battery voltage VBAT,COMP at a constant 28.3 volt level, as indicated by line 20 of FIG. 1. Thus, battery voltage, VBAT,COMP, which is continually sensed by the alternator regulator as an analog voltage signal via the external remote sensing wire, is maintained at a constant, desired 28.3 volt level that matches VSet0.
In other words, in the example prior 24-volt battery charging system having external remote sensing capabilities, VCONTROL,COMP at 100% alternator load, is 28.5 volts (i.e., VSet0+0.2 volts). At less than 100% alternator load, the alternator's internal control point voltage VCONTROL,COMP linearly decays to 28.3 volts, the original VSet0 value at 0% alternator load. Referring to FIG. 2, linear scaling of the change in voltage control point VCONTROL,COMP along line 22 to offset or compensate for battery cable voltage losses helps keep the voltage at the battery, VBAT,COMP, substantially constant at the 28.3-volt level indicated by line 20 of FIG. 1. Hence, the example prior charging system having external remote sensing capabilities offsets or compensates for the linearly varying battery cable voltage losses over the entire range of alternator loads or duty cycles.
In contradistinction, some other prior battery charging systems altogether lack remote sensing capabilities. Battery charging systems lacking external remote sensing capabilities are, in comparison to those having external remote sensing capabilities, relatively less expensive and entail relatively less wiring complexity, but in such systems battery cable losses are not offset and are uncompensated for. As a result, battery voltage VBAT,UNCOMP linearly decreases with increases in alternator load. As is well-known in the art, alternators used in prior charging systems lacking remote sensing capabilities are often provided with internal voltage sensing and control capabilities through which the alternator output voltage setpoint VCONTROL,UNCOMP at the B+ terminal can be held at a prescribed, constant level over the entire alternator load range. For example, VCONTROL,UNCOMP can be constantly maintained at the alternator's prescribed VSet0 level over the entire alternator load range.
As discussed above, battery cable voltage losses linearly increase with increasing alternator load, and in the present example of a charging system lacking remote sensing capabilities, the uncompensated battery voltage VBAT,UNCOMP linearly decreases from 28.3 volts at 0% alternator load, which matches the alternator's prescribed VSet0 level, by about 0.2 volts as the alternator load is increased to 100%. The linear drop in the voltage supplied to the battery with increasing current draw through the battery cable is indicated by line 30 of FIG. 1, which shows battery voltage VBAT,UNCOMP decreasing linearly with increasing alternator load, from 28.3 volts (i.e., the VSet0 level) at 0% alternator load, to about 28.1 volts at 100% alternator load. Meanwhile, referring to line 32 of FIG. 2, the alternator output voltage setpoint VCONTROL,UNCOMP at the alternator B+ terminal is maintained at its constant VSet0 level of 28.3 volts over the entire alternator load range between 0% and 100% in this example.
In other words, because the voltage drop through the battery cable connecting the alternator B+ terminal and the battery increases linearly with alternator output current, in a battery charging system lacking remote sensing capabilities wherein the alternator's output voltage setpoint at the B+ terminal (VCONTROL,UNCOMP) is maintained constant at the alternator's VSet0 level, and the actual battery voltage VBAT,UNCOMP drops linearly with increasing alternator load. In the present example, referring to line 30 of FIG. 1, at the 0% alternator load level, the alternator's substantially unloaded state at which cable voltage losses are negligible, VBAT,UNCOMP is 28.3 volts, the alternator's prescribed VSet0 level. But as battery cable voltage losses increase linearly with alternator load, reduced voltage levels are delivered to the battery. VBAT,UNCOMP drops to 28.1 volts at 100% alternator load, or 0.2 volts below the level of 28.3 volts at the 0% alternator load level, although the alternator's internal voltage sensing and control capabilities maintain VCONTROL,UNCOMP at the B+ terminal constant at the VSet0 level of 28.3 volts over the entire range of alternator loads. Thus, while charging systems altogether lacking remote sensing capabilities provide relatively lower cost and complexity than prior battery charging systems having external remote sensing capabilities, the latter system type provides relatively better charging performance.
Modern vehicles typically include a controller area network (“CAN”) through which different ECUs for various subsystems send and receive messages to, for example, control subsystem operations or receive feedback from sensors. The ECUs form nodes which serially communicate through the CAN bus. Each node is able to send and receive messages, but not simultaneously, and the serial communication of the messages is on a priority basis. While there may be no delay to higher priority messages which are immediately retransmitted, the refresh rate for lower priority messages can be as long as one to three seconds.
In some modern vehicles having a CAN, information identifying the sensed battery voltage level VBAT is already communicated on the CAN bus. If such a vehicle's battery charging system also has external remote sensing capabilities, it is often seen as cost-advantageous to omit the alternator regulator's analog signal receiving input pin and the dedicated external remote sensing wire connecting the input pin and the positive terminal of the battery, and replace the alternator controller with one receivable of the serially communicated information identifying the battery voltage level VBAT. Thus, serial communication to an alternator controller of battery voltage information over a vehicle CAN may facilitate some ability to provide the alternator regulator and/or alternator controller (hereinafter the alternator regulator/controller) with remote battery voltage sensing signals for use in regulating alternator output, and avoid at least some of the costs and wiring complexity associated with a battery charging system having external remote sensing capabilities.
However, depending on messaging priority levels, serial communication speeds, and CAN bus loading, it may not be possible to send a signal indicative of the sensed battery voltage over the CAN bus at a rate fast enough for the alternator regulator/controller to efficiently regulate the alternator output voltage setpoint. In other words, the vehicle's serial communication protocol may be too slow to adequately provide data through the battery charging system control loop to maintain voltage supplied to the battery at a desired level, such as, for example, the constant VBAT,COMP level provided by the prior system having external remote sensing capabilities indicated by line 20 in FIG. 1. Additional issues with relying on serial communication over the CAN bus to maintain battery voltage at a desired, constant level arise when the vehicle serial communications are set up to communicate the constant voltage level desired at the battery (e.g., 28.3 volts in the present example), but not the compensating voltage adjustments to VCONTROL by which the varying battery cable losses between the alternator B+ terminal and the battery are offset. Heretofore, the performance of prior external remote sensing systems has not been conveniently and inexpensively matched by utilizing VBAT information serially communicated over the vehicle CAN bus to the alternator regulator/controller.
Although analog inputs could be implemented to serially communicate the battery voltage signal(s) to an analog input of the alternator regulator for monitoring the battery voltage, such an arrangement would undesirably increase wiring complexity and charging system costs. On the other hand, heretofore, relying on serial communications to achieve any real-time feedback of sensed battery voltage to the alternator regulator/controller without providing external remote sensing capabilities requires providing faster, more expensive serial communication networks, or assigning higher messaging priority to battery voltage information, neither of which is desirable. A method or system for achieving the performance levels associated with external remote sensing capabilities in a battery charging system, without providing the external remote sensing capabilities themselves or a relatively more expensive and/or faster serial communication network, or assigning higher serial communication messaging priority to sensed battery voltage information, is desirable.